Report connection errors on streams as HTTP/3 errors

Motivation:

When the peer closes the connection, SwiftNetwork delivers the
CONNECTION_CLOSE to every stream flow, so each stream's pipeline sees a
'QUICConnectionError' before the connection's pipeline does. The
coordinator's own fan-out ('cancelStreamsDueToConnectionClose') can't
reach those streams because they have already closed and deregistered by
the time it runs.

'HTTP3StreamHandler' passed the error through untouched, so the owner of
a request stream saw a QUIC error rather than an HTTP/3 one. It already
translates the other two peer-close errors, RESET_STREAM and
STOP_SENDING; CONNECTION_CLOSE was the missing third.

Modifications:

- Translate 'QUICConnectionError' into an 'HTTP3Error' with code
  'remoteConnectionError' in 'HTTP3StreamHandler.errorCaught', keeping
  the peer's reason phrase and, for application errors, its error code.

Result:

Streams report HTTP/3 errors when the connection is closed by the peer

Author: glbrntt

Sources/NIOHTTP3/HTTP3StreamHandler.swift

@@ -302,6 +302,20 @@ package final class HTTP3StreamHandler: ChannelDuplexHandler {
             case .none:
                 break
             }
+        case let error as QUICConnectionError:
+            self.logger.trace("Caught CONNECTION_CLOSE")
+            // RFC 9114 § 8: "Receipt of an unknown error code MUST be treated as equivalent to
+            // H3_NO_ERROR."
+            let h3Code = error.isApplication ? HTTP3ErrorCode(rawValue: error.code) ?? .H3_NO_ERROR : nil
+            context.fireErrorCaught(
+                HTTP3Error(
+                    code: .remoteConnectionError,
+                    message: error.reason,
+                    cause: error,
+                    errorCode: h3Code,
+                    location: .here()
+                )
+            )
         default:
             context.fireErrorCaught(error)
         }